The present invention relates to a heat-deformable, austenitic nickel-chromium-iron alloy with very high oxidation resistance and thermal strength.
Such alloys are used for production of wires and bands for heating conductor-resistors, for production of support systems in ovens, as well as for other oven parts, and in increased volumes also for core reactors.
An alloy for support systems in ovens is disclosed for example in the German document DE-PS 3,037,209 and has the following composition:
8 up to 25% chromium PA0 2.5 up to 8% aluminum PA0 0.005 up to 0.04% yttrium PA0 up to 15% of one or several elements Mo, Rh, Hf, W, Ta and Nb PA0 up to 0.5% of one or several elements of C, B, Mg, Zr and Ca PA0 up to 1% Si, up to 2% Mn, up to 20% Co, up to 5% Ti, up to 30% Fe, the rest Ni. PA0 0.01 up to 0.5% C PA0 0.01 up to 2% Si PA0 0.01 up to 3% Mn PA0 22 up to 80% Ni PA0 10 up to 40% Cr PA0 0.0005 up to 0.20% B and/or PA0 0.001 up to 6% Zr as well as PA0 0.001 up to 0.5% Ce and/or PA0 0.001 up to 0.2% Mg and/or PA0 0.001 up to 1% Be PA0 max. 0.15% C PA0 max. 0.3% Al PA0 14 up to 19% Cr PA0 max. 0.5% Cu PA0 19 up to 25% Fe PA0 max. 2.0% Mn PA0 0.5 up to 2.0% Si and PA0 at least 59% Ni (including 1% Co). PA0 up to 0.08% C PA0 0.1 up to 0.2% Al PA0 14.0 up to 16.0% Cr PA0 up to 0.5% Cu PA0 19.0 up to 23.0% Fe PA0 0.1 up to 0.8% Mn PA0 1.1 up to 1.6% Si PA0 0.001 up to 0.04% Ca PA0 up to 0.05% N PA0 up to 0.01% S PA0 up to 0.015% P PA0 0.01 up to 0.04% lanthanide as cerium-misch metal PA0 rest nickel. PA0 17 up to 25% Fe PA0 14 up to 20% Cr PA0 0.5 up to 2.0% Si PA0 0.1 up to 2.0% Mn PA0 0.04 up to 0.10% C PA0 0.02 up to 0.10% Ca PA0 0.010 up to 0.080% N PA0 0.025 up to 0.045% Ti PA0 0.04 up to 0.17% Zr PA0 0.03 up to 0.08% Y PA0 less than 0.010% S PA0 less than 0.015% P PA0 each less than 0.1% Mo, W, Co PA0 each less than 0.05% Nb, Ta, Al, V, Cu PA0 rest nickel
Thereby first of all a highly adhesive aluminum oxide layer is obtained, which preferably is produced by preoxidation in oxygen-containing atmosphere at 1093.degree. C. An aluminum content of 2.5-8% produces in this alloy however a strong .gamma.'-separation, preferably in the temperature region of 600-800.degree. C. This is connected with a strong ductility reduction of the material, and in the ovens which often during heating and cooling pass this temperature region, can lead to material damages.
Moreover, the aluminum contents of 2.5 to 8% at chromium contents of 8 to 25% are not sufficient to form exclusively aluminum oxide in NiCrAl-alloys. Furthermore, for formation of aluminum oxide, chromium oxide, mixed oxides and inner oxidation, a process is used which especially at temperature-cyclical loads lead to a worsened protective action than the pure chromium oxide.
Another heat resistant and highly thermally deformable alloy is disclosed in U.S. Pat. No. 3,865,581 and has the following composition:
rest iron.
In accordance with claim 2 of this patent, the alloy can contain also Ti, Al and Y.
By the dosed addition of B, Zr, Ce, Mg and Be, the number of effectually exceeding torsions at 1050 to 1300.degree. C. is considerably increased, therefore it can directly connected with the improvement of the thermal deformability. In this alloy it was considered as disadvantageous that the improvement of the thermal deformability detected in short time torsion test leads to burdens of long time properties such as for example oxidation strength. So it is for example known that B, Mg and Be worsen the oxidation properties of the material by modification of the oxide layer during thermalcycical oxidation. The positive action of cerium was lost at temperatures above 1200.degree. C by the formation of a low-melting eutektic. The positive influence of zirconium on the oxidation strength is neutralized when zirconium for improvement of the thermal deformability is present as stabile carbide. Moreover, the positive influence of zirconium on the thermal deformability properties can be reversed when coarsely dispersed separated zirconium carbide forms by not adjusted zirconium and carbon admixtures.
Finally, DIN 17,742 (Material No. 2.4867) discloses an alloy with
This alloy is produced in form of wires and bands for manufacturing heat conductors and electrical resistors. It is produced and sold with the following composition:
These heating conductor alloys are shortly identified as NiCr 60 15. It has under the temperature alternate load (in accordance with FIG. 1b, so below) the service life lying between that of the pure NiCr-alloy NiCr 80 20 on the one hand, and that of the iron-base material NiCr 30 20 on the other hand (see FIG. 2). Moreover, the alloy NiCr 60 15 despite its higher melting point has a lower maximum use temperature than the pure NiCr alloy and has no sufficient thermal strength or certain applications.